In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners, containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor's surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor's surface. A set of fuser rollers or belts, under heat, melts and fixes the toner in the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an intricate and ongoing series of steps in which the photoconductor's surface is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor's surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is critical to making the process work.
After the image is transferred to the paper or other recording medium, it goes to the fuser where the paper is moved through a nip where it is heated and pressed. This melts the thermoplastic portion of the toner, causing it to intermingle with the fibers of the paper, thereby bonding the image onto the paper. While this is an effective way of fixing the toner image on the paper's surface, it carries with it some negative consequences. Specifically, various types of copy media, such as bond paper and tracing paper, contain significant amounts of moisture. During the passage of the paper through the fusing area, this moisture is heated and evaporates. The steam vapor can then escape into other portions of the printer creating the potential for rust and corrosion, which can inhibit long-term machine performance. The steam can condense and form puddles in entrapment areas. The moisture can also condense on the surface of the back-up or pressure roller in the fuser. When it does so, it is carried around to the fuser nip, reducing the coefficient of friction between the back-up roller, the paper and the fuser belt, causing the paper to slip. This slippage delays the arrival of the paper at the exit sensor, registering as a paper-feed failure, causing the machine to stop. In another scenario, the slippage of the belt, caused by moisture in the fuser area, causes the paper to not enter the fuser nip thereby producing a fuser jam. In both cases, the printer ceases operation, requiring that the operator clear and restart it, delaying completion of the printing project underway.
The problems caused by moisture are particularly acute where the printer utilizes a fuser belt, rather than a fuser roll, especially one which is not self-driven, but rather is driven by friction between the belt, the paper and the back-up roller (which is driven). In this commonly used apparatus, when moisture condenses on the back-up roller, it wets the fuser nip and the fuser belt. This can result in slippage between the fuser belt, the paper and the back-up roller which delays arrival of the paper at the exit sensor, causing the printer to stop. This requires that the operator clear the paper path and restart the printer in order to complete the print job. Another problem caused by the presence of moisture is the result of back-up roller/fuser belt slippage. Such slippage can cause a paper bubble, as the paper enters the fuse nip, which not only can result in a paper jam, but can also cause the paper to rub against fuser surfaces, causing the unfixed toner to be smeared. As used herein, these problems are collectively referred to as "fuser stalls."
It is clear that, for several reasons, it is important to effectively remove moisture, created by the fusing process, from the back-up roller in the belt fuser. The present development defines an effective way to accomplish this goal.
Although the prior art recognizes that the production of moisture by the fusing process is undesirable, there are few methods suggested for combating this problem and those methods which have been suggested have significant drawbacks associated with them.
U.S. Pat. No. 5,223,902, Chodak, et al., issued Jun. 29, 1993, describes a moisture collection and removal system for a fuser. The fuser involved does not use a back-up or pressure roller, but rather forms a fusing nip between the fuser roller and a pad biased against the fuser roller. In this system, moisture condenses and falls by gravity into a collection area; a wiper is not used. The printing apparatus described is relatively large, such as those used for making blueprints. This large size provides a significant amount of space which may be used for dealing with the moisture problem. Large amounts of space are not available in a desktop printer or copy machine, making it much more of a challenge to deal with the moisture issue.
U.S. Pat. No. 4,822,978, Morris, et al., issued Apr. 18, 1989, describes a fuser apparatus which utilizes a low-mass fuser roller and a flexible web to keep sheets of paper in biased contact with the fuser roller. The web contains perforations that allow accumulated moisture to escape from the fuser system; the moisture can then be wiped from the outer surface of the web. There is no back-up roll utilized in this system and no structure is given for the wiping mechanism. Again, this apparatus is suggested for use in a relatively large printer and would not be useful in a smaller desktop model with its associated space constraints.
U.S. Pat. No. 4,645,327, Kimura, issued Feb. 24, 1987, describes an apparatus for preventing condensation of moisture on the surface of a photoconductor. It does not address the issue of moisture in the fusing system. This patent describes (see column 10, lines 31 et. seq.) a wiper comprised of an aluminum shaft having layers of felt and/or urethane sponge to wipe moisture off the photoreceptor drum. Such a wiper structure is not generally effective in dealing with the moisture problem, since it tends to absorb water until it becomes saturated, at which time it begins feeding water back onto the surface of the photoreceptor.
U.S. Pat. No. 5,307,133, Koshimizu, et al., issued Apr. 26, 1994, addresses the problem of moisture condensation on the fuser apparatus by incorporating a fan into the printer to eliminate water vapor in the air. This is an indirect way of addressing the problem that is not as effective as directly addressing the issue of moisture accumulation on the back-up roller.
U.S. Pat. No. 5,091,752, Okada, issued Feb. 25, 1992, addresses the moisture condensation issue by incorporating a heat-insulating surface layer on the back-up roller. This approach requires reformulation of the rollers in the printer. It would be highly desirable to be able to effectively address the moisture condensation issue without having to significantly modify the structure of the rollers.
Concurrently-filed U.S. patent application Ser. No. 09/491,278 Hamilton, et al., Back-up Roll with Reduced Mass, describes a back-up roller used in the fusing portion of an electrophotographic process, comprising an inner metal core and an outer hollow shell surrounding the core, with a plurality of metal ribs between the core and the shell. This roller reduces the condensation of moisture on its surface.
It has now been found that moisture accumulation on the back-up roller can be minimized or eliminated by utilizing a wiping element, such as a brush, made from a high-surface energy material, in contact with the surface of the back-up roller. This approach not only effectively removes moisture from the back-up roller, thereby eliminating stalling, paper jams and corrosion of parts, but it achieves those ends effectively, inexpensively, in a manner suited to the small spaces available in a desktop printer, and without requiring redesign of the fuser belt and back-up roller.